Between full side Uzi, Mini UZI and MP5 SD (these are the 3 that I have shot), I like MP5 SD, because of the smoothness and controllability. I imagine MP5 is the same. Wonder if MP5K PDW is the same.  Anyone knows?

Having had the oppertunity to shoot many of the models listed, there's no question that the HK MP5 series is the easiest and smoothest SMG.  Firing from a closed bolt makes all the difference.  With lots of practice, you can learn to fire some of the others nearly as well, but the MP5 is hard to beat.

I've never had the chance to fire an MP5SD, but that's what I'd buy if I could, and could afford to.

-Troy

[b][/b]


 My guess:
 1.  The weight factor.
 2.  Uzi is open bolt; therefore, first shot is
     less accurate.
 3.  MP5 is more accurate when go full auto.
 4.  Probably, cool factor.

 I shot all three. And I like them all.

Tough question.  First off, I'm probably a little biased b/c I instruct with the Uzi for our [URL=http://www.frontsight.com/1day_smg_course.htm]Free Uzi Course[/URL], but I'll pitch in my $.02.

The Uzi offers a bunch of benefits:

1.) With the stock collapsed it is easily concealable, and can be deployed very quickly to its fully extended position.

2.) A lower cyclic rate (~600 RPM) than the MP-series offers much easier trigger modulation to deliver a specified number of shots on demand.  One shot for a head-shot is very easy with a little training, versus one from an MP5.  

For full auto, your target is not going to be impressed that you engaged him with 600 vs. 1200 RPM after 2-3 rounds have been delivered.

3.) Relative weight is negligible compared to some of the other heavier weapons systems (someone mentioned this)

4.) Price.  Often overlooked for us civilians who have to buy Class III.  You can get a nice new Vector Uzi for $3,000-3,400 or build a parts gun for less.  Not many Class III weapons with the benefits and reliability of the Uzi are in this price range.

Some other points:

Open Bolt Accuracy - Yes, technically OB weapons systems are inherently less accurate, buy given the proper grip and stance this is negligible for the Uzi.  The Uzi was designed as a primary battle rifle, and its range out to 300 yards suffers more from other factors (i.e. the shooter and the cartridge being fired) than from being open bolt.  In CQB this is totally negligible for the trained individual.

Choice of Spec Ops, etc - A lot of politics and personal preference is involved in branches of service choosing weapons.  Let me point out that the Uzi is the primary weapon of the Secret Service who have one of the most important duties in the world.  If you remember the attempted assassination on Reagan, there's footage of a SS Agent producing an Uzi out of thin air.

[UZI]

Train like you fight and you'll fight like you've trained!

 I should replace "cool factor" to Ergonomics like RedGoat stated.
 M4 : As far as cool factor is concerned, I should a [:D] to state my intention.  Actually, if everything is equal between the 2 (Uzi and MP5), I probably take MP5 or SD.